Television signals originating from a satellite are amplified and converted into a predetermined frequency band (typically 950-2150 MHZ) using a parabolic dish and a low-noise converter located at the focus of the parabolic dish. This signal is then directed to the input of the tuner of the receiver. The purpose of the tuner is to select the desired channel and to output a baseband signal on the in-phase path (I path) and on the quadrature path (Q path). This signal is then converted into a digital signal and demodulated.
The channel decoding processing comprises a block which distinguishes the logic zeros from the logic ones, typically by means of majority logic. The block also performs the entire error correction using Viterbi decoding, deinterlacing, Reed-Solomon decoding or deshuffling, for example. The channel decoding device outputs packets which are decoded in a conventional manner in a source decoding device in accordance with the MPEG standards to redeliver at the output the initial audio and video signals transmitted via the satellite.
The tuner is associated with a local oscillator which generates the quadrature radio frequency signals. It is vital to control this quadrature, since any quadrature errors between the signals arising from the oscillator and delivered to the mixers of the tuner leads to an inaccuracy in the decoding of the digital information received. Also, control of the quadrature must be performed over the entire frequency of the signals received. By way of indication, the development specifications specify a phase shift error that shall remain less than a few degrees, such as 3°, for example.
Within the field of portable telephones, the transmission of data between a portable telephone and its base is not continuous, but is in the form of packets. Also, between the transmission of these packets, it is possible for the portable telephone to recalibrate the quadrature of the local oscillator by injecting a test signal, for example.
However, such calibration is not possible in digital television. This is because the information is transmitted continuously and no time is available to perform any calibration of the local oscillator. One approach includes acting directly on the local oscillator to improve its quality. However, this poses significant technological problems, thus having a direct impact on manufacturing costs.
Moreover, the doctoral thesis presented by Patricia Coget (12 Jul. 1989, University of Limoges) entitled “Design of a broadband microwave monolithic 0-90° phase shifter for radio beams”, discloses a broadband monolithic 0-90° phase shifter integrated on an ASGA substrate for a radio receiver. To operate this circuit over a broad frequency band, use is made of a slaved system that includes a controllable phase shifter, a phase comparator, an amplifier and a loop filter.
However, such a circuit has numerous drawbacks. One of these drawbacks resides in the structure of the phase shifter. The phase shifter is formed of capacitive resistive cells, thus requiring very accurate matching of the components when constructing the phase shifter. Moreover, the local oscillator described in this prior art document delivers two signals in phase opposition from which are formed four signals mutually in quadrature. However, if a phase shift error exists between the two signals delivered by the local oscillator, this phase shift error also persists between the two phase opposition signals delivered by the phase shifter, and also between the other two phase opposition signals delivered by the phase shifter.
Furthermore, the slaving according to this prior art comprises a single control of the phase shifter. This is very penalizing with regards to the elimination of any noise. Finally, such a circuit is not fully integrated and, in any event, does not lend itself to complete integration on a silicon substrate because such a substrate has a propensity to propagate noise.